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Although it would've stood only waist-high next to a human, Hypsilophodon foxii could run away in a flash, and now new research suggests it even may have possessed a special adaptation that prevented its ribs from rattling during Olympic-worthy dashes.

The discovery, which has been accepted for publication in the journal Cretaceous Research, yields both bad and good news for the plant-eating dinosaur.

The bad is that scientists now believe it did not possess impressive body armour, as had previously been suspected.

The good is they now think the thin mineralised plates that had earlier been identified as evidence for armour were actually cartilage tissues that may have helped to regulate breathing, especially during periods of extreme physical exertion.

"Hypsilophodon had elongated legs and a stiffened counterbalancing tail that suggest it was almost certainly a fast runner," says Richard Butler, who co-authored the study with Peter Galton.

Overlaid not fused

He and his colleague had multiple skeletons to analyse, since the dinosaur, whose name means "high-crested tooth," was one of the earliest ever discovered, having been named in 1869.

Based on multiple specimens from southern England, primarily the Lower Cretaceous Wealden Group region of the Isle of Wight, it's believed this was one of the most common dinosaurs in the area from around 132 to 125 million years ago.

Butler and Galton noticed that the supposed body armour on the dinosaur actually consisted of weakly constructed plates that overlaid, but were not fused to, the ribs.

Butler says that other scientists assumed "that as the body of the dead Hypsilophodon individual rotted and collapsed...the bones from the skin came to be closely associated with the internal bones."

"Our careful re-examination of the specimens shows, however, that the bony plates are always closely associated with the outside surface of the ribs from the front end of the ribcage and are certainly not armour," he says.

Bird-like ribcage

Butler and Galton instead believe the plates were similar to bony structures, called "uncinate processes," seen in the ribcages of birds today.

Since this feature is involved in bird ribcage support, facilitating movement and breathing, the scientists now suspect the structures played a similar role in the dinosaur.

Since other research shows Hypsilophodon could run on its two legs at close to 22 kilometres per hour, or about the same speed as a long-distance Olympic runner, extra support for such huffing and puffing events would seem warranted.

Because evidence for similar mineralised plates has been detected in other dinosaur skeletons, the researchers further theorise that all small-bodied, bird-footed dinosaurs possessed these structures.

Clint Boyd, who also studies dinosaurs and is a doctoral candidate at North Carolina State University, says, "I absolutely agree with their interpretation that these structures are intercostal plates and not dermal armour."

Boyd says that because many other dinosaurs may possess this feature, the discovery "has opened up an exciting new area of research into (the plates') evolution and function."

Not unique

Darren Naish, an honorary research associate at the University of Portsmouth, is also intrigued by the fact that the structures could be "widespread, possibly even universal" in other small, herbivorous dinosaurs.

Naish says that Hypsilophodon "has undergone quite a change in appearance since its initial discovery."

"It was first imagined to look rather like a large, climbing lizard," he says, but when later research found it was "an agile, terrestrial runner...it always seemed odd that a fast runner might retain rows of armour plates."

Naish adds, "People have mostly forgotten about these 'armour plates' in recent decades, but this new work shows that a fresh look at old specimens can still reveal new information on even the best known of dinosaurs."